This invention is an improvement in vanes for small wind turbines. It applies to small exchange of momentum wind turbine vanes.
Wind energy has been utilized by mankind since ancient times. It is believed that mechanical windmills in one form or another have been in use for thousands of years. Horizontal axis windmills have been in use over 700 years. Modern technology and innovation are making the gathering of wind energy much more efficient and wind energy devices more self-regulating.
Windmills can be classified by the orientation of the axis on which the blade or blades are mounted, specifically, horizontal axis and vertical axis wind turbines. One of the advantages of some vertical blade wind turbines is that they do not need to be oriented with respect to the direction of the wind because they are designed to utilize wind from all directions.
Modern, small, horizontal-axis wind turbines usually have a blade assembly upwind of the tower supporting the wind turbine and a tail assembly extending downwind of the tower. The tail assembly orients the wind turbine with respect to the wind. A typical small horizontal axis system has a generator, power train, and blade assembly mounted so that they can rotate as a unit about a vertical axis allowing them to track the wind. Larger, more sophisticated, and more expensive wind turbines have devices which can determine the direction of the wind and control mechanisms powered by electric motors which turn the entire unit to face into the wind.
There are some wind turbines, on the other hand, that have the blade assembly downwind of the tower upon which they are mounted. The oncoming wind produces a force on the blade system which tends to orient the blades to be perpendicular to the wind. The main drawback to such systems without the stabilizing effect of a tail assembly is the tendency of the entire wind turbine to make wide rotational excursions about its vertical axis due to the randomness of wind interacting with the turbine's blades. This is particularly disruptive for exchange of momentum machines since their vanes have much more surface area than those of wind turbines using Bernoulli's Principle.
There are two types of horizontal axis windmill blade systems: the aforementioned, now ubiquitous Bernoulli's Principle type and the exchange of momentum type. Exchange of momentum devices have been in use for thousands of years. Windmills operating on Bernoulli's Principle are relatively new. They have come to dominate in the marketplace despite the superiority—at least for small units—of windmills operating on the exchange of momentum principle. Both types have their advantages. Bernoulli's Principle wind turbines are a better choice in the case of higher speed wind and in the case of very large wind turbines. Exchange of momentum systems are better suited to low-speed wind and lower power, less expensive, smaller generator systems.
Although exchange of momentum wind turbines have many advantages over Bernoulli's Principle turbines, there is one case in which Bernoulli's Principle turbines are clearly superior and the only reasonable choice, specifically, for turbines of very large size. In the case of large diameter turbines, the speed of the blades at their extremities is so great that energy cannot be transferred practically by the means of exchange of momentum, and the induced drag force is so large that the energy lost from drag forces greatly overwhelms any exchange of momentum blade system. Some Bernoulli's Principle wind turbines have blades in excess of 180 feet in length. At a leisurely pace of just 10 revolutions per minute, the speed of the tips of blades of such length will exceed 125 miles per hour. At such speeds, the vane of an exchange of momentum system would be retreating from the oncoming wind at such a high speed that an extremely high velocity wind would be necessary just to keep up with the retreating vane surface—much less transfer momentum to the vane. Additionally, if the vanes of an exchange of momentum system could reach such speeds, drag forces would greatly overwhelm an exchange of momentum system.
The blades of wind turbines operating on Bernoulli's Principle have a profile that is similar to that of the wings of an aircraft. Their driving force is the same force that causes an airplane wing to produce lift. The faster the wind blows, the greater the “lift” force exerted on the blade. This greater force moves the blade faster and produces more power. However, the faster the blade moves, the greater the drag force acting on the blade which tends to slow it down, decrease its efficiency, and reduce the power it can produce.
The speed at which the end or tip of a vane moves depends upon the rate of rotation of the vane and its length. For any given speed of rotation, the tip of the vane moves the fastest and each incremental portion of the vane towards the hub moves slower. The speed at which the vane tip of a smaller wind turbine vane advances is not great even for a relatively fast turning vane. Consider and compare a small vane turning twice as fast as the large blade in the example given above. A vane having a length of 10 feet turning at 20 revolutions per minute (twice as fast as the example above) has a vane tip which is moving at only slightly over 14 miles per hour. If the vane angle is 30 degrees, the vane surface at the end of the vane is moving at only about 7 miles per hour in the direction of the axis of rotation. If the wind is also moving at 7 miles per hour, the wind is exerting no force on the vane in the direction of its axis of rotation. Hence, any wind that is moving at a speed in excess of 7 miles per hour would contribute power.
The vane systems of exchange of momentum turbines typically have a very large surface area compared to Bernoulli's Principle blades. The greater the area of the vane, the greater is the potential exchange of momentum and, therefore, power. However, concomitant with that larger vane surface area is the possibility of extreme forces acting on the vane system when it is subjected to very high-speed winds.
One of the great problems with wind turbines and particularly exchange of momentum wind turbines is the potential significant damage high-speed wind can inflict upon the device. This is particularly true with exchange of momentum wind turbines with their vanes having a very large surface area exposed to high-speed wind. The forces produced by high-speed winds impacting the relatively large surface area of exchange of momentum turbine vanes can become extremely destructive.
Many means have been developed to protect wind turbine blades from the extremely destructive forces of high-speed winds. One method illustrated in U.S. Pat. No. 1,936,233 (Groves, 1933) has a wind-driven rotor behind a series of vertically mounted shutters which can be closed in front of the turbine. Closing the shutters blocks the wind from reaching the turbine. This system has significant drawbacks, however, such as requiring a very high-strength tower and massive tower foundation to resist the extremely high wind forces the system would experience in a strong storm. These forces would be particularly large if the shutters were closed thereby not allowing wind to pass through. In addition, the many slats in the shutter system would be susceptible to icing and could fail to operate in cold and icy weather.
U.S. Pat. No. 4,177,012 (Charles, 1979) reveals another way turbine blades could be protected from high-speed wind. This patent discloses a fan blade for an automobile having a series of bends which would decamber with increasing fan speed. Such a device could be adapted for use as a wind turbine. However, such an arrangement would have at least one very significant disadvantage in a wind turbine. Because winds are uneven and constantly varying in speed and force, wind turbine blades constructed in this fashion would be continually cambering and decambering. The constant changes in shape and mass distribution while rotating could cause large, unpredictable forces in the relatively large wind turbine blade system and supporting tower which could lead to its destruction, or, at the very least, require significant design effort in an attempt to anticipate and design for such forces. U.S. Pat. No. 3,758,231 (Barnstead, 1973) is similar in having the deflection of the blade more or less proportional to the force applied to it and, therefore, suffering from the same disadvantages of possible cycling of shape and mass distribution if it were to serve as a wind turbine blade.
Another way to counteract the destructive forces of high-speed winds which has been designed for wind turbines incorporates mechanisms to “feather” the vanes or sails as they are sometimes called. U.S. Pat. No. 1,334,485 (Clipfell and Manikowske, 1920) is illustrative. The wind turbine in this case features a multiplicity of vanes radially disposed about a central hub. The wind turbine is designed to allow the vanes to rotate about their own longitudinal axes under the influence of a plurality of governor weights which react to the speed of the rotor. Thus, the vane could be oriented along the flow of air for minimal interaction with the wind when the wind speed is high or the vanes could be turned so the flow of air would make contact with the vane's surface to cause exchange of momentum when the wind speed is less. Unfortunately, the large number of vanes with accompanying hinges, pins, and connecting arms would be susceptible to corrosion and, most significant, freezing in winter. Such an eventuality could cause the unit to be destroyed in high winds. Likewise, U.S. Pat. No. 2,633,921 (Monney, 1953) incorporates a flexible canvas-like vane secured by cables and tensioning springs to a number of “shafts” extending radially from a hub. This device, too, suffers from its susceptiblity to malfunction caused by extreme weather and ice.
U.S. Pat. No. 4,632,637 (Traudt, 1986) discloses a mechanism which allows the blades of a downwind turbine to both feather and pivot downwind so that, instead of extending radially in possibly damaging high winds, the blades pivot back so they extend downwind and out of the flow of the wind. This would be, if it were built, a complex and expensive device which would also be susceptible to icing and would be unable to produce any power after the blades had pivoted.
In addition to the applicable aforementioned impairments, Bernoulli systems require higher windspeeds to start their relatively narrow blades systems in rotation from rest than do exchange of momentum systems. Their slender and expensive blades are only able to accept a small percentage of the kinetic energy of the air that passes through their swept area—a percentage far less than the theoretical Betz Limit—thereby diminishing their potential power output. Furthermore, Bernoulli Principle blades must have carefully designed and precisely manufactured blades in order to implement the Bernoulli Principle. Such precision makes them much more expensive to manufacture than exchange of momentum vanes.
Indeed, Bernoulli Principle blades are very expensive to use in wind turbine applications. While Bernoulli Principle wind turbines would extract more power if they were equipped with three, four, or even five blades than they would with just two blades, the very high cost of the blades makes it too expensive to produce Bernoulli Principle wind turbines with more than three blades since each additional blade costs as much as the previous blade but adds less power because of increased parasitic drag. Vanes constructed in accordance with the invention disclosed herein need merely be a relatively flat surface. They need not have special characteristics beyond being substantially impermeable to air.